1. Field of the invention
This invention relates to thermoelectric elements which are efficient at relatively high temperatures for the measurement of temperature and, more particularly is concerned with semiconductor materials useful for such elements.
2. Background of the prior art
Thermocouples have been used for many years as accurate temperature measurement sensors. In fact, the International Practical Temperature Scale IPTS-68 is based on the type S platinum plus 10% platinum-rhodium reference grade thermocouple for temperatures above 950K. With the development of thin film techniques it was recognized that there are several advantages of thin film thermocouples over the standard wire thermocouples for measuring solid surface temperatures. Their low mass leads to very rapid response, the thinness of the film (-2 .mu.m) does not disturb the convective heat transfer characteristics of the surface compared to the wire sensor, and the small size is quite compatible with complex electronic fabrication techniques. These advantages have led to development of numerous thin film thermocouple applications based on standard thermocouple alloys including temperature sensors for interior hardware in gas turbine and internal combustion engines.
Previous work by Bates et al, Am. Cer. Bull., 65(4), 1986, 673, had indicated a strong negative Seebeck coefficient in bulk In.sub.2 O.sub.3 -SnO.sub.2. The Seebeck effect, which causes an electrical current flow when the junctions of dissimilar materials forming a thermocouple loop are subjected to different temperatures has long been known and has been utilized, as in thermocouples, for measuring temperatures. More recently, devices utilizing two unlike semiconductor materials in a thermocouple loop have been found to have low, but, for some purposes, useful efficiencies in the generation of electrical power from heat applied to one junction of the materials. However, such devices have shown marked decreases in efficiency as the temperature of the heated junction was raised and no material capable of providing a reasonably efficient thermoelectric device for operation at temperatures in excess of about 1,000.degree. K. has previously been known.
The intent of this invention is to provide a practical thin film thermocouple using reactively sputtered thin film. Thin film thermocouples for high temperature measurement can be found in U.S. Pat. No. 3,006,978 to McGrath et al which discloses thin film thermocouple conductors which permit the use of brittle thermocouple materials capable of withstanding the effects of 5000.degree. F. temperatures. Further disclosures may be found in U.S. Pat. No. 3,758,346 to Falkenberg et al, a thermoelectric generator; U.S. Pat. No. 4,665,276 to Ebel et al, a thermoelectric sensor; and U.S. Pat. No. 4,779,994 to Diller et al, a heat flux gage. None of the above patents disclose the transparent electrical conductor indium-tin-oxide (ITO) or any transparent material. ITO is an oxide containing 5-15 percent by weight tin in the indium oxide with the most useful compositions containing an In:Sn ratio of 9:1.
Other prior art devices may be found in U.S. Pat. No. 4,010,291 to Katsube et al; U.S. Pat. No. 4,399,015 to Endo et al; and U.S. Pat. No. 4,450,316 to Hamakawa et al. Although the prior art devices disclose indium-tin-oxide as a transparent electrode or a two-layer (SnO.sub.2 and ITO) transparent electrode, there is no mention of the thermoelectric effect of ITO or SnO.sub.2.